The present invention relates to wooden structural members which can be used as beams, joists, studs, posts or the like.
Hitherto, where savings in weight or material cost is of importance, conventional lumber which is rectangular in cross-section, has been substituted by wooden I-beams or X-beams.
For example, a wooden I-beam is disclosed in U.S. Pat. No. 4,191,000 issued Mar. 4, 1980--L. R. Henderson. The tongue-and-groove connections between the central web and two outer flanges of this type of wood I-beam may limit the allowable design load of the beam given certain beam width and depth dimensions. This is due to the strength capacity of the web and flange connections, which is determined mainly by the amount of contact area available between the web and the flange, which is usually limited. Moreover, the shear strength capacity of typical tongue-and-groove connections in wooden I-beams cannot be readily increased, unless the design of the connection is itself altered.
A further drawback characteristic of wooden I-beams is differential thickness swelling, and which is encountered between the flange and web at the tongue-and-groove connection. When moisture is absorbed, the web member tongue swells, whilst the groove in the flange member shrinks; and vice versa, when moisture is desorbed. Differential lateral movement repeated over time will result in separation of the glue bond between the tongue and the groove, or in splitting of the flange members at the tongue-and-groove joint location.
Christ-Janer in U.S. Pat. No. 4,446,668 issued May 8, 1984 discloses a novel wooden X-beam construction which employs, in cross-section, two webs which are centrally joined together along their length and which have disposed therebetween, top and bottom wedge shaped flanges. The Christ-Janer form of X-beam construction is very restricted in terms of manufacturing flexibility. Sizes of beams are not easily changed, as each time the configuration is changed, a new moulded shape of the web members, which accommodate the flanges, is required. Problems due to thickness swelling or shrinkage also exist, since the opposed web members, which are centrally connected, resist lateral movement resulting from swelling or shrinkage of the wedges disposed above and below the central connection of the two webs. As a result, lateral swelling of the wedge flange members exerts a tensile stress on the connection between the two web members. Conversely, lateral shrinkage puts a tensile stress on the connection between the flanges and the web members.